A fuel cell is an energy converting device which directly converts chemical energy of fuels directly into electrical energy, and due to its eco-friendly characteristics of high energy efficiency and low release of pollutants it has been developed as a next generation energy source. A proton exchange membrane fuel cell also known as polymer electrolyte membrane fuel cell (PEMFC), which contains polymer electrolyte membrane polymer, has been highlighted as a power device for portable use as well as uses in vehicles and at homes for advantages such as low operating temperatures, elimination of leakage problems caused by the use of a solid electrolyte, and fast operations.
A polymer electrolyte fuel cell, being a type of a direct current power generation device converting chemical energy of fuels directly into electrical energy by an electrochemical reaction, is a continuous complex consisting of a membrane-electrode assembly (MEA), which serves as the heart of a fuel cell, and a bipolar plate, which collects electricity generated and supplies fuels. In particular, the membrane-electrode assembly refers to an assembly of an electrode, in which an electrochemical catalytic reaction occurs between a fuel (an aqueous methanol solution or hydrogen) and air, and a polymer membrane in which, a hydrogen ion transfer occurs.
Meanwhile, all electrochemical reactions are divided into two separate reactions, that is, an oxidation, which occurs at the anode and a reduction, which occurs at the cathode. The anode and cathode are separated by an electrolyte. In a direct methanol fuel cell among fuel cells, methanol and water are supplied to the anode instead of hydrogen, and the hydrogen ions generated from the oxidation reaction of methanol move to the cathode along with the polymer electrolyte and generate electricity by a reduction reaction with oxygen supplied to the cathode. The details of the reactions are as follows:Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2OOverall reaction: CH3OH+3/2O2→CO2+2H2O
An ion exchange membrane used as a solid electrolyte in a fuel cell is present between the two electrodes and transports hydrogen ions produced in the oxidizing electrode (anode) to the reducing electrode (cathode).
Generally, the electrolyte membrane used in a polymer electrolyte fuel cell can be classified into perfluorinated and hydrocarbon polymer electrolytes. The perfluorinated polymer electrolyte has been commercially available as a polymer membrane of polymer electrolytic fuel cells, because it is chemically stable due to a strong carbon-fluorine (C—F) bonding force and shielding effects, a characteristic of a fluorine atom, has an excellent mechanical property, and especially an excellent conductivity as a hydrogen ion exchange membrane. Nafion (a perfluorinated sulfonic acid polymer), a product of a U.S. company, Du Pont, is a typical example of commercialized hydrogen ion exchange membrane, and is most commonly used because of excellence in ionic conductivity, chemical stability, ionic selectivity, etc. However, the perfluorinated polymer electrolyte membrane, with such superior performances has disadvantages of a low industrial utilization due to a high price, high methanol crossover, the permeability of methanol to pass through the polymer membrane, reduced polymer membrane efficiency at 80° C. or higher. Thus, studies have been focused on the development of a competitive hydrocarbon ion exchange membrane.
Polymer electrolyte membranes used in fuel cells must be stable at conditions required during a fuel cell operation. Thus, the polymers to be used are very limited to those such as an aromatic polyether (APE), etc. Hydrolysis, oxidation, and reduction reactions, etc., during a fuel cell operation reduce fuel cell performance by decomposing a polymer membrane. Thus, polyetherketone-based or polyethersulfone-based polyaryleneether polymers have been studied for their application in fuel cells, because of their excellent chemical stability and mechanical properties.
As a method for producing a polymer with improved ion conductivity, a polymer introduced with a hydrophilic functional group has been used. U.S. Pat. No. 4,625,000 discloses a post-sulfonation process of polyethersulfone as a polymer electrolyte membrane. The post-sulfonation method disclosed in the patent has a drawback that it is difficult to control distribution, location and number of sulfonic acid groups (—SO3H), because it uses a strong acid such as sulfuric acids as a sulfonating agent and sulfonic acid groups are randomly introduced into the polymer backbone.
Further, European Patent No. 1,113,517 A2 discloses a block copolymer electrolyte membrane consisting of blocks with or without sulfonic acid groups. Because a block copolymer consisting of an aliphatic block and an aromatic block is post-sulfonated using sulfuric acid, a strong acid, there are problems such as decomposition of chemical bonds between aliphatic polymers, etc., and also difficulties in controlling location and number of sulfonic acid groups in the polymer backbone due to random introduction of sulfonic acid groups into the ring constituting an aromatic block.
Meanwhile, Japanese Patent Application Publication No. 2003-147074 discloses a method of introducing a sulfonic acid group into a copolymer containing fluorine compound using chlorosulfonic acid (HSO3Cl) or sulfuric acid. In the method, Sulfonic acid groups are randomly introduced onto a ring constituting a fluorene compound.
Polymer sulfonation methods proposed in the related arts were unable to satisfy the physical properties for electrolyte membranes required during a fuel cell operation because they excessively increase water and methanol contents of electrolyte membranes and thus dissolve electrolyte membranes in methanol, etc., when the degree of sulfonation (DS) is increased in order to achieve a hydrogen ion conductivity similar to that of a commercially available Nafion (Dupont), thus significantly reducing the degree of mechanical integration of electrolyte membranes.
Meanwhile, in order to solve the problems of fossil fuel exhaustion and environmental contamination, efforts such as reducing consumption of fossil fuels by improving their use efficiency or extending the applications of renewable energies into an increased number of fields have been made. Although renewable energy sources, such as solar and wind power, are more efficiently used than ever before, these energy sources are not still prevalent and unpredictable. Due to such characteristics, there is a limited dependence on these energy sources, and the proportion of the renewable energy sources in the primary power source is very low.
A rechargeable battery provides a simple and efficient way to store electricity, so efforts have been continuously made to increase its mobility by minimizing its size, thus utilizing it as an infrequent auxiliary power supply or as a power supply for laptops, tablet PCs, mobile phones and other small electric appliances.
A redox flow battery (RFB) is a secondary battery which stores energy for a long period by repeated charging and discharging respectively via electrochemical reversible reactions of electrolytes. Because stacks and electrolyte tanks which respectively control the capacity and output characteristics of the battery are conFIG.d to be independent from each other, the designing of the battery can be performed without limitation, and also there is little limitation regarding space for its installation.
The redox flow battery exhibits a load leveling function, compensates or inhibits functions for power failure or instantaneous low-voltage, etc., which enables accommodating a rapid increase in electricity demand by installing them in power plants, power systems or buildings. The redox flow battery is a very powerful energy storage technology which can be combined freely as necessary and also a system suitable for large-scale energy storage.
Such Redox flow battery consists of two separated electrolytes. One electrolyte stores electroactive materials from the negative-electrode reaction while the other is used in the positive-electrode reaction.
In the real redox flow battery, electrolytes reactions in the cathode and the anode differ from each other, generating a pressure difference between the cathode and the anode due to the flow phenomenon of an electrolytic solution In an all-vanadium redox flow battery, a representative redox flow battery, the reactions of catholyte and anolyte may be expressed as follows:

Therefore, to overcome the pressure difference between both electrodes and to enable an excellent cell performance regardless of repeated charging and discharging, a separation membrane with improved physical and chemical durability is needed. However, when the thickness of a separation membrane is increased to improve the physical durability, it results in an increased resistance.